This Invention relates to stimulation of wells drilled into subterranean formations to improve recovery of hydrocarbons. In particular it relates to hydraulic fracturing and acid fracturing of low permeability producing zones to improve the flowpath available to oil and gas flowing to the wellbore. Most particularly it relates to using less expensive chemicals, less complicated procedures, and less hydraulic horsepower to create fractures that have greater width and higher conductivity than would be produced by other methods when fracturing comparable formations.
Stimulating the production of hydrocarbons (or other fluids such as water, brine, and carbon dioxide) is well known. Hydraulic fracturing (pumping a fluid into a wellbore at a pressure and rate sufficient to split open the formation rock), acidizing, or a combination of the two (called acid fracturing or fracture acidizing) are the most common techniques. In hydraulic and acid fracturing, a first, viscous fluid called a xe2x80x9cpadxe2x80x9d is typically injected into the formation to initiate and propagate the fracture and often to contribute to fluid loss control. This is followed by a second fluid. In hydraulic fracturing the second fluid contains a proppant that keeps the fracture open after the pumping pressure is released. In acid fracturing, the second fluid contains an acid that can dissolve part of the rock, causing irregular etching of the fracture face and removal of some of the mineral matter, resulting in the fracture not completely closing when the pumping is stopped. The choice of the pad fluid depends upon the nature of the subsequently injected fluid and of the formation and on the desired results and attributes of the stimulation job.
Occasionally, hydraulic fracturing is done without a highly viscosified second fluid; this choice is made primarily as a way to reduce the deleterious effect of polymers described below. This technique, sometimes called a xe2x80x9cwater-fracxe2x80x9d involves using extremely low polymer concentrations, so low that they cannot be effectively crosslinked, throughout the job. This alternative has a major drawback: since there is inadequate viscosity to carry much proppant, high pump rates must be used and only very small concentrations (pounds mass proppant added per gallon of fluid), called xe2x80x9cPPAxe2x80x9d, of proppant can be used. Very little proppant will be placed in the fracture to keep it open after the pumping is stopped.
Pads and fracturing fluids are usually viscosified in one of three ways. If the injected fluid is an oil, it is gelled with certain additives designed for the purpose, such as certain aluminum phosphate compounds. If the fluid is water or brine, for hydraulic or acid fracturing, it is gelled with polymers (usually crosslinked with a boron, zirconium or titanium compound), or with viscoelastic fluids (xe2x80x9cVES""sxe2x80x9d) that can be formed using certain surfactants that form appropriately sized and shaped micelles. Polymers, especially crosslinked polymers, often tend to form a xe2x80x9cfiltercakexe2x80x9d on the fracture face, that is they coat out on the fracture face as some fluid leaks off, provided that the rock pores are too small to permit entry of the polymer or crosslinked polymer. Some filtercake is generally desirable for fluid loss control. This process of filtercake formation is also called wallbuilding. VES fluids do not form filtercakes as a result of leak-off. VES leak-off control is viscosity controlled, i.e., the resistance due to the flow of the viscous VES fluid through the formation porosity limits the leak-off rate. The viscosity controlled leak-off rate can be high in certain formation permeabilities because the highly shear-thinning fluid has a low apparent viscosity in high flow velocity areas. Reducing the flow velocity (by correspondingly reducing the pressure gradient or simply as a result of the same injected volumetric flow rate leaking off into the formation through a greater surface area as the fracture grows in length and height) will allow micelle structure to reassemble and will result in regeneration of viscosity and fluid loss control. On the other hand, polymers have two major deficiencies: a) the filtercake, if left in place, can impede subsequent flow of hydrocarbons into the fracture and then into the wellbore, and b) polymer or crosslinked polymer will be left in the fracture itself, impeding or cutting off flow, either by physically blocking the flow path through the proppant pack or by leaving a high viscosity fluid in the fracture. VES fluids do not form a filtercake or leave solids in the fracture. VES fluids therefore leave a cleaner, more conductive and therefore more productive fracture. They are easier to use because they require fewer components and less surface equipment, but they may be more expensive and less efficient than polymers, depending upon the formation permeability and the specific VES system and polymer system.
For expediency, pads are typically chemically similar to the fracture fluids used in the same job. Thus in hydraulic fracturing with water-based crosslinked polymer-containing fluids, the pad is typically a water-based crosslinked or uncrosslinked polymer. Crosslinking decreases the amount of polymer needed but increases complexity and often the cost of the job and usually leaves a more harmful residue on the fracture face or in the fracture after the job. In hydraulic fracturing with water-based VES fluids, the pad is typically also a water-based VES fluid, but all of this fluid leaks off, and it may be expensive.
U.S. Pat. No. 5,036,919 (Ronnie L. Thomas and Curtis L. Boney, issued Aug. 6, 1991), hereby incorporated by reference, describes the benefit and importance of a method of fracturing designed to minimize damage to the fracture proppant pack conductivity by using a less damaging, less thermally stable, viscosifier in the fracture fluid than in the pad. In that case, the pad was an organo-metallic crosslinked polymer-based system and the fracture fluid was preferably a borate-crosslinked polymer-based system.
Whereas, the pad is typically the same chemical system (albeit at different concentrations as the proppant-laden or acid containing stages), optimal performance may be realized by using substantially different fluid chemistry or fluid types for both the pad and proppant-laden or acid containing stages. The pad need not be chemically the same as the fluid used in the later stages since the pad does not need to transport proppant or acid and the stimulation plan may include greater pad leak-off than leak-off of subsequent fracture stages. There is a need in hydraulic and acid fracturing to provide an inexpensive material for the pad that will a) create and propagate a sufficiently wide fracture, b) minimize chemical, equipment and hydraulic horsepower expense, c) form a filtercake to control leak-off during the job (not absolutely needed in all cases, for example not required when acid fracturing with a low viscosity acid) and d) be degradable to maximize subsequent production.
In low permeability formations being fractured with VES fluids, a low concentration (such as about 10 to about 15 ppt (pounds per thousand gallons fluid)) of uncrosslinked polymer may be used in the pad and provide an inexpensive and effective material for the pad that will a) create and propagate a sufficiently wide fracture, b) minimize complexity and chemicals, equipment and hydraulic horsepower requirements, c) form a filtercake to control leak off during the job (not absolutely needed in all cases, for example not required when acid fracturing with a low viscosity acid) and d) be degradable to maximize subsequent production. This optimizes the pad and the fracture fluid separately for low permeability formations. The viscosity of the uncrosslinked polymer may actually be lower than that of the VES fluid subsequently pumped, but the uncrosslinked polymer is less expensive than crosslinked polymer or than VES, less complicated to use and requires fewer chemicals and less equipment than crosslinked polymer, and, in low permeability formations, will still form a filtercake to increase fracture generation efficiency. Either the pad or the subsequent fracture fluid or both can be foamed or energized, if the surfactants selected to form the VES are known to create stable foams, or if the fluids further comprise foamers and the VES surfactants selected are not anti-foamers. The pad and subsequently pumped fracture fluid may have all the additives typically used in such fluids. The job may be a hydraulic fracture, an acid fracture or a fracture immediately followed by gravel-packing, for example with a screen in place, in a single operation. It is a particularly important aspect of the Invention to employ these fluids and methods in fracturing jobs in which only a small amount of proppant is used (commonly called xe2x80x9cwaterfracsxe2x80x9d or xe2x80x9cwater-fracsxe2x80x9d or xe2x80x9cslickwater fracsxe2x80x9d. It is another particularly important aspect of the Invention that the methods allow pads ahead of VES fracture fluids to be pumped with minimized hydraulic horsepower, number of pumps, etc. relative to conventional fracturing with pad compositions previously known in the art for VES based fluids. It is another aspect of the Invention to generate a higher conductivity fracture than if crosslinked polymer or a high concentration of uncrosslinked polymer was used in the pad. It is yet another aspect of the present Invention to provide sufficient fracture width and fluid loss control while minimizing damage to the proppant pack when proppant is used. A particularly preferred embodiment of the Invention is a method of stimulating production of fluids from subterranean formations penetrated by a wellbore involving injecting a first fluid (the pad), made from an aqueous carrier fluid containing an uncrosslinked polymer at a concentration of less than about 15 pounds per thousand gallons fluid, and then injecting a second fluid containing a thickening amount of a viscoelastic surfactant. The key unifying element of the methods of the Invention is a low concentration of an inexpensive, easy to use uncrosslinked polymer used in the pad to minimize damage due to the pad while then subsequently using a fracturing fluid that is completely non-damaging but nonetheless has excellent proppant carrying properties.